As is known in the art, a modulator-demodulator (modem) is an electronic device that modulates transmitted signals and demodulates received signals. The modem generally provides an interface between an analog communications system and digital devices to thus make possible analog transmission of digital information between two terminals or stations. Such transmissions may be over a transmission line, such as a telephone or other communication line for example, coupled between the two stations. Such transmission lines are generally band-limited. That is, the information may be transmitted over the transmission lines only over a predetermined range of frequencies. For example, telephone transmission lines are generally provided having a bandwidth of 3 kilohertz (kHz). Thus, the speed with which the modem may transmit and receive data is limited, in part, due to bandwidth limitations of the transmission medium.
As is also known, an analog signal representing, e.g. a voice signal, may be represented as a stream of bits (i.e. a digital signal). The digitized signal, which may be converted back to an analog representation of the digital data for the purpose of transmission over a telephone lines, is less susceptible to noise on the telephone line, is capable of multiplexed channel operation in the telephone bandwidth, reduces crosstalk and enables relatively easy digital encryption for secure transmission.
The digitized voice signal, which typically is at, e.g., 64,000 bits-per-second (bps), may not be readily sent by modem within the available approximately 3,000 Hz bandwidth of the telephone lines or be readily sent at that bit rate within the available telephone line bandwidth. To enable more convenient transmission and/or multiplexed transmission, compression of the 64,000 bps digitized voice signal is generally employed. For example, with compression from 64,000 bps to 2,400 bps, four simultaneous channels of 2,400 bps each may be multiplexed via a modem onto a 9,600 bps data stream transmitted over the bandwidth of the analog telephone lines.
As is also known in the art, encryption of analog signals such as voice signals, for example, may be accomplished via analog or digital techniques. Analog techniques include frequency inversion and other operations (e.g. modulation) on the amplitude, frequency or phase of the time varying analog signal. Such operations may or may not be accomplished with a digital computer.
Digital voice encryption techniques, including the American National Standard Institute's (ANSI) data encryption standard (DES) algorithm, perform bit stream operations on digitized samples of the analog signal and then transmit the digital result to a receiver. Such transmission may be accomplished through a modem for example. To use a modem for transmission of a speech signal, the speech signal is compressed. That is, some of the input information (i.e. the input speech signal) is discarded such that the signal may be transmitted at a rate at which the modem operates. This results in a loss of information which may manifest itself as a degradation in voice sound quality.
The processing required to perform the compression and expansion of digitized voice results in an undesirable delay and consequently in a two line transmission system, for example, objectionable echo signals result. Furthermore, the inefficiency of modems limits the rate at which a modem may transmit and receive signals fed thereto. Thus it would be desirable to provide a system which minimizes such processing time and thus minimizes the echo signal and also maximizes the rate at which a modem may transmit and receive data over a transmission link.
Thus, at least several problems have tended to impair the effectiveness of conventional limited bandwidth, e.g., telephone bandwidth, analog and digital voice transmission apparatus.